8.19 - Nature, nurture and brain evidence

Inherited information and experience

Genes help build a brain that can respond to experience, while experience helps shape how its circuits develop and function.

Genes are expressed in developing cells. Their products help control processes such as neurone growth, signalling and synapse formation. Differences in alleles can therefore contribute to differences in brain structure or responses to an environment. This does not mean that an allele specifies a particular memory or fixes a person's eventual ability.

An environmental stimulus can produce activity in a developing neural pathway. Activity can influence which connections are strengthened or retained; less-used connections may be weakened or lost. As you saw for visual development, the timing of experience matters because some circuits are especially sensitive during a critical period. Environmental conditions can also affect gene expression. An inherited capacity to respond and the experience needed to develop that response can both be necessary.

An interaction means that the effect of one factor depends on the other. For example, inherited differences might affect how strongly a developing pathway responds to a particular experience; the outcome would then depend on both genotype and the experience received. It is therefore usually misleading to ask whether an entire brain is caused by nature or nurture. A better question is which genetic and environmental differences contribute to a specified developmental outcome under specified conditions.

Researchers must define that outcome. The volume of a brain region, its response to a stimulus and performance on a memory task are different measurements. Behaviour can provide evidence about brain function, but a test score is not a direct measurement of a synapse or of the whole brain.

What newborn abilities can reveal

Testing babies soon after birth reduces the amount of postnatal learning that could explain an observed ability.

Newborns cannot describe their perceptions, so researchers measure a response such as sucking, looking time or a change in heart rate. A controlled comparison asks whether responses differ between two stimuli. For example, in DeCasper and Fifer's 1980 study, sucking patterns controlled whether a newborn heard its mother's voice or another woman's voice. The babies produced the mother's voice more often. This demonstrated early discrimination and preference; it did not identify a gene for recognising that particular voice. Primary study.

The strength of this method is that an ability is demonstrated before extensive experience after birth. Reflexes such as sucking also show that some functional neural circuits are present very early. However, present at birth does not mean produced by genes alone. The fetus has already developed in an environment and can experience sound before birth. Research comparing filtered and whispered maternal voices supports a role for acoustic information available prenatally in early voice recognition. Primary study.

A sound-preference investigation should use comparable sound levels, counterbalance the stimulus order or response–sound association, and compare many babies rather than rely on one. Otherwise a louder recording, an easier sucking pattern, fatigue or chance could explain a preference. Parental consent, safe sound levels and stopping when a baby is distressed are essential. Deliberately depriving infants of normal care or sensory experience would be unacceptable.

The response is also an indirect measure: longer looking could indicate novelty rather than liking, and a baby who shows no preference might be tired or unable to make the measured response. Positive and negative results both need interpretation. Repeated observations at different ages can distinguish an early capacity from later improvement, although increasing age brings both maturation and additional experience.

Animal experiments and causal evidence

An experiment can change one aspect of an animal's experience while controlling other conditions, providing stronger causal evidence than simply observing different people.

In Hubel and Wiesel's visual-deprivation work, researchers restricted visual input through one eye during early life and recorded responses of cells in the visual pathway. Many cortical cells subsequently responded predominantly to the non-deprived eye. Comparison with normal visual experience, and with deprivation at different ages, helped establish that early input affects development of visual function. Primary cortical-recording study.

The inference is that changed sensory input can alter the development of a genetically developing visual system. The experiment does not show that genes are irrelevant: the eyes, neurones and capacity for activity-dependent change still develop through biological processes involving gene expression. It also does not show that all human learning has the same critical period as kitten vision.

Controlled animal work allows age, diet, housing, duration of exposure and the type of stimulus to be standardised. Where appropriate, random allocation helps distribute pre-existing differences across conditions. A comparison group exposed to the same handling and procedures, except for the factor being investigated, helps distinguish that factor from effects of stress or handling. Replication across animals addresses biological variation; many recordings from one animal do not replace independent animals.

For an enrichment study, merely comparing a spacious cage containing toys and companions with an empty solitary cage would change several factors. A difference could be due to exercise, social contact, sensory stimulation or stress. A more focused comparison separates those factors, while maintaining acceptable welfare, and measures a defined outcome using observers unaware of group membership where possible.

Animals share features of neural development with humans, so these studies can suggest mechanisms that human observations can test. Species, developmental timing and living conditions differ, which limits transfer. Harmful deprivation studies are historical evidence, not classroom procedures. Ethical justification must weigh expected benefit and harm, consider non-animal alternatives, minimise numbers consistent with valid evidence and refine procedures to reduce suffering.

Learning from damaged brain areas

Studying people with brain damage can reveal which structures and connections contribute to a function without deliberately causing that damage for research.

Researchers compare a person's abilities before and after an injury or medically necessary operation, when reliable earlier records exist. They can also compare patients with different damage and people without that damage, using similar tests and brain imaging to locate affected tissue. A selective loss of one ability with other abilities relatively preserved is more informative than a general fall in performance.

Scoville and Milner reported that H.M. had a severe difficulty retaining new experiences after surgery affecting structures in both medial temporal lobes, while general intelligence was relatively preserved. This supported a contribution of that region to forming lasting memories. It did not show that every kind of memory was lost or that all memories are stored in a single place. Primary case report.

A lesion can damage several structures or interrupt connections to distant areas. Later anatomical work on H.M. identified both surviving hippocampal tissue and other pathology, illustrating why precise anatomical verification matters. Primary reconstruction study. Performance can also be affected by the original illness, medication, fatigue and previous experience. Small, unusual case samples may not represent everyone.

These studies contribute to nature–nurture investigations by showing the neural machinery needed for learning and how experience after injury can contribute to recovery or compensation. Plasticity is the capacity of neural structure or function to change. If performance improves during rehabilitation, altered use of surviving pathways may contribute; spontaneous recovery, time since injury and repeated practice on the test are alternative explanations that need comparison. Recovery does not establish that the damaged tissue has regrown.

Damage studies therefore constrain explanations of normal function, but they do not directly reveal exactly how an undamaged brain originally developed. Loss of a function after damage supports a contribution by the affected system; it does not prove a simple one-region–one-function map. Multiple cases, accurate imaging, repeated measurements and comparisons with other methods strengthen the inference. Participation also requires appropriate consent, protection from distress and respect for patient privacy.